Or, they may have actually tested raid rebuild on a system that did not have a single write during the rebuild and only with fresh drives - under this very spec

Or, they may have actually tested raid rebuild on a system that did not have a single write during the rebuild and only with fresh drives - under this very specific use case, if the RAID controller does things right, all writes are sequential from the first disk sector until the last -- which is a use case that SMR doesn't slow down in; essentially, the only such use case.